Injection molding control method

ABSTRACT

The control cycle for an injection molding machine comprises successive fill, compaction, hold, cure and mold open times. The ram position and mold pressure are monitored at predetermined times respectively after the molding cycle has begun. If the ram position is out of tolerance, the fill pressure is adjusted to compensate for the deviation. If the mold pressure is out of tolerance, the compaction pressure is adjusted to compensate for the deviation in mold pressure.

BACKGROUND OF THE INVENTION

In injection molding of thermoplastic material it is desirable that theparts produced be uniform in shape, size, weight, strength andappearance. These characteristics are partly dependent upon theproperties of the material and the design of the mold, but they are alsothe result of the control exercised over the molding operation. Suchcharacteristics are dependent upon the temperature of the material, therate of flow of the plasticized material into the mold and the densityof the material in the mold.

The temperature of the plasticized material, or melt, being injectedinto the mold must be maintained within its working range. The rate offlow of the plasticized material into the mold determines the flowpattern in the mold, how well the mold is filled and the orientation ofthe molecules of the material. The rate of flow is dependent upon theviscosity of the melt and the pressure applied to it, the viscosity, inturn, being inversely related to temperature. The density of thematerial in the mold is dependent upon the complete filling of the mold,the viscosity of the melt in the mold and the pressure applied to it.

There is a fundamental relationship between the pressure, volume andtemperature of a plasticized material expressed in the Spencer andGilmore equation:

    (P + π) (V - ω) = RT

in which

P = plastic pressure

V = plastic volume

T = plastic temperature

π = constant for the plastic

ω = constant for the plastic

R = constant for the plastic

It will be seen that a change in any variable will result in a change ofat least one other variable, so that manipulation of one variable may beemployed to compensate for changes in another variable.

During the primary injection portion of the molding cycle, the primaryinjection pressure supplied to move the ram has been held substantiallyconstant. As the ram is moved forward by the primary injection pressurethe ram is moved at a substantially constant speed while the melt isinjected through a nozzle into the mold cavity until the cavity issubstantially filled, being limited only by the rate of flow of the meltthrough the gate, which rate is dependent upon melt viscosity. After themold cavity has been initially filled, the ram continues to moveforward, but at a greatly reduced speed, as the melt is compacted in thecavity, until the mold pressure plus the pressure drops in the flowpassages equals the pressure exerted by the ram on the melt, at whichtime forward movement of the ram ceases. The mold pressure builds upslowly until the cavity is initially filled, after which it rapidlyincreases while the melt is being compacted until it equals the pressureexerted by the ram.

Without any control, if the viscosity of the melt had increased, as inresponse to a variation in composition, the opposing pressure on the ramwould increase, slowing flow of the melt into the mold cavity and soreducing the time for compaction. The melt begins to set-up as soon asit enters the mold. With the slower flow the melt has a longer time toset-up before the cavity is filled and so builds up a greater resistanceto compaction, the result being that the peak mold pressure is lower.Finally, as a result of the lower peak pressure, the subsequent partswill be smaller and lighter than before. If the viscosity of the melthad decreased, the cavity would fill faster, the melt would not set-upas much before the cavity was filled, the compaction time would belonger, the peak pressure would be higher, and the subsequent partswould be larger and heavier. The change in length of the finished partsis due to resilience when the pressure is removed, while the change inweight is due to different densities resulting from the difference incompaction.

Many different systems have been employed in efforts to provide the mostconsistent results at the lowest possible price. When manual controlsare employed, the quality of the product is dependent upon theexperience and skill of the operator. Automatic controls usually producemore uniform quality throughout a production run and also from run torun. Some automatic controls are so simple that they do not producesatisfactory results, while others are so complex they cannot beeconomically justified. Better and less expensive controls are alwayssought.

Some automatic controls have controlled the speed of the ram as itinjects the melt into the mold cavity by adjusting the primary injectionpressure. This compensates for changes in viscosity while the mold isbeing filled, but the adjusted pressure is also employed to compact thematerial in the mold. If the melt viscosity has increased, the primarypressure is adjusted upward to compensate for the resulting reducedflow. This will typically produce more dense, heavier and longer partsthan without controls. Conversely, if the melt viscosity decreases, theprimary pressure is adjusted downward typically producing less dense,lighter and shorter parts than without the control. Controls dependentsolely upon ram speed tend to overcompensate for viscosity changes.

Other automatic controls have controlled the pressure in the filled moldcavity by adjusting the primary injection pressure, but the adjustedpressure is also employed to inject material into the mold during thefollowing molding cycles. An increase in mold pressure at apredetermined time in the primary injection cycle indicates a decreasein melt viscosity, while a decrease in mold pressure indicates anincrease in melt viscosity. If the melt viscosity has increased, themold pressure will decrease, requiring a compensatory upward adjustmentof primary injection pressure and typically resulting in production ofshorter and lighter parts than without the control. Conversely, if themelt viscosity has decreased, the mold pressure will increase, requiringa downward adjustment of primary injection pressure and typicallyresulting in production of longer and heavier parts. Mold pressurecontrol thus tends to under-compensate for viscosity changes.

SUMMARY OF THE INVENTION

This invention provides a method for automatically controlling aninjection molding machine to consistently produce molded parts ofsatisfactorily uniform quality by a relatively simple method. It dividesthe primary injection cycle into two periods - a fill period and acompaction period. It monitors ram position at a predetermined timeafter initiation of a primary injection cycle and while the mold cavityis being filled as an indication of the viscosity of the melt flowinginto the mold cavity. If the sensed position deviates from apredetermined set point, the injection pressure applied to move the ramduring the fill period, hereinafter referred to as fill pressure, isadjusted to compensate for the difference in viscosity. At a secondpredetermined time after initiation of the primary injection cycle andafter the mold cavity has been filled, but before the mold pressure issubstantially reduced, it monitors mold pressure as an indication ofdensity of the plasticized material in the cavity. If the sensed moldpressure deviates from a predetermined set point, the injection pressureapplied to move the ram during the compaction period, hereinafterreferred to as compaction pressure, is adjusted to compensate for thedifference in density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a primary injection pressure control systemaccording to this invention combined with a schematic diagram of aninjection molding machine and the hydraulic control circuit employed toprovide injection pressure.

FIG. 2 is a graph of ram position plotted against time during a typicalprior art injection molding cycle.

FIG. 3 is a graph of mold pressure plotted against time during a typicalprior art injection molding cycle.

FIG. 4 is a graph of ram position and mold pressure with their setpointlimits plotted against time according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a typical injection molding machine, a hydraulic controlcircuit employed to provide injection pressure and that portion of anautomatic control therefor exercising primary injection cycle controlaccording to this invention. The molding machine 10 comprises a mold 11having a cavity 12 therein to receive a plasticized molding material.Particulate molding material 13 is fed by gravity from a hopper 14 intoan extrusion barrel 15. A combination screw and ram 16 is rotatable inthe barrel by a motor 17 to plasticize the molding material by shearingand working it, thereby generating heat. An hydraulic injection cylinder18 at the rear end of the barrel has a piston 19 therein movable byhydraulic pressure to move the ram 16 lengthwise in the barrel. When theram is moved forward by the piston, plasticized molding material, ormelt, is injected into the mold cavity 12 through a nozzle 20.Conventional machine timers 22, 23, 24 and 25 control the primaryinjection, secondary injection, cure and mold open sequence. The timersare triggered by a mold lock-up signal from the machine 10. We areconcerned solely with the primary injection period.

A continuously running pump 26 delivers a hydraulic fluid 27 from a sump28 through a flow restricting valve 29 to cylinder 18 behind piston 19so as to move the piston forward. The fluid delivered by the pump is ata relatively high pressure, which is regulated by a normally openelectro-hydraulic pressure relief valve 30, controlling return of thepressurized fluid to the sump in well-known manner.

A position transducer, shown as a potentiometer 31, is coupled to theram 16 to provide a position signal responsive to ram position. Mostmolding machines have a position indicator to which the wiper 32 of thepotentiometer may be connected. A mold pressure transducer 33, such as astrain gauge, is located in communication with the mold cavity 12 toprovide a signal responsive to mold pressure. The mold pressure could besensed in the mold cavity 12, in the gate 12A, or in the sprue andrunner system. As shown the mold pressure is transmitted to the pressuretransducer 33 through a dummy ejector pin 34.

A position comparator 35 is connected to receive a variable ram positionsignal from the position transducer 31, a predetermined constantsetpoint signal from an adjustable ram position setpoint 36 and aposition sample time signal from a position sample timer 37. Thecomparator delivers a ram position deviation signal as a function of thedeviation of the sensed ram position from the setpoint position and of apolarity corresponding to the direction of the deviation when theposition sample timer delivers its signal at a predetermined time afterit receives the mold lock-up signal and while the cavity 12 is beingfilled with the melt. A fill pressure control 38 sums the ram positiondeviation signal and a fill pressure reference signal from a fillpressure reference 39 to provide a fill pressure control signal.

A pressure comparator 40 is connected to receive a variable moldpressure signal from the pressure transducer 33, a predeterminedconstant pressure setpoint signal from an adjustable mold pressuresetpoint 41 and a pressure sample time signal from a pressure sampletimer 42. The pressure comparator delivers a mold pressure deviationsignal as a function of the deviation of the sensed mold pressure fromthe setpoint pressure and of a polarity corresponding to the directionof the deviation. The pressure sample timer delivers its signal at apredetermined time after it receives the mold lock-up signal from themachine 10 and after the cavity 12 has been filled with the melt, butbefore the pressure in the mold is substantially reduced. A compactionmold pressure control 43 sums the pressure deviation signal and acompaction pressure reference signal from a compaction pressurereference 44 to provide a compaction pressure signal.

A process timer 45 is energized during the primary injection period by asignal received from the primary injection timer 22. It divides theprimary injection period into a fill period and a compaction period.During the fill period it delivers the fill pressure signal to aprogrammable power supply 46 and during the compaction period itdelivers the compaction pressure signal to the programmable powersupply. The programmable power supply delivers a valve control signalresponsive to the received fill and compaction pressure signals to theelectrohydraulic pressure relief valve 30 to control the hydraulicpressure supplied to cylinder 18 during the primary injection period,and thereby to adjust independently the injection pressures on theplasticized material during the fill and compaction portions of theprimary injection period as required to produce the desired ram positionand mold pressure at the respective predetermined times.

FIGS. 2 and 3 show variations in ram position and mold pressure withtime during a typical injection molding cycle according to the priorart. The zero time is determined by the mold lock-up signal providedwhen the halves of the mold are clamped together and the cavity is incondition to receive the melt. The primary pressure on time isdetermined by the primary injection timer 22, the secondary pressure ontime by the secondary injection timer 23, the cure time by the curetimer 24 and the mold open time by the mold open timer 25.

During the primary pressure on time a predetermined high hydraulicpressure regulated by pressure relief valve 30 is delivered to thecylinder 18, moving the piston 19 and connected ram 16 forward, therebyinjecting melt from extrusion barrel 15 through nozzle 20 into cavity12. The time is set long enough to permit complete filling of the cavityand compaction of the melt in the cavity. It is generally set to allowmold pressure to attain its maximum value for any reasonable variationin melt viscosity. It will be seen in FIG. 2 that with a low meltviscosity the ram completes its stroke filling the mold cavity in lesstime than with a high melt viscosity. FIG. 3 shows that with a low meltviscosity a higher mold pressure is obtained than with a high meltviscosity. In order to optimize production the primary pressure on time,as well as other cycle times, is kept as short as possible.

During the secondary pressure on time the hydraulic pressure in cylinder18 is usually reduced by pressure relief valve 30 to a predeterminedholding pressure to substantially prevent back flow of the melt. Thetime is set to permit solidification, or curing, of the melt in the gateto such an extent that the melt will not flow out of the mold when thepressure is removed. The ram does not substantially move during thistime, but the mold pressure decreases as the temperature of the meltdeclines and melt in the cavity begins to cure. FIG. 2 shows thesubstantially steady ram positions, and FIG. 3 shows the progressivelydecreasing mold pressures with passage of time. The mold pressure curvesare similar for both melt viscosities, but are offset due primarily tothe difference in peak pressures. Ideally the mold pressures should fallto zero during this time as the high melt viscosity curve does.

During the cure time hydraulic pressure is released from the cylinder 18so that the ram 16 is permitted to move backward and all mold pressureis removed. When the secondary injection pressure is released, the motor17 begins to rotate the ram 16, plasticating by shearing and working themolding material 13, flowing by gravity from hopper 14 into theextrusion barrel 15, as the ram turns. The ram, acting as a screw, movesthe material forward in the barrel, building up a pressure in theforward end of the barrel that forces the ram backward to its initialrearward position. This backward movement is generally opposed by apredetermined back pressure in cylinder 18. When it has moved into thisinitial position the motor 17 stops rotating and plastication iscomplete. The cure time extends beyond the completion of plasticationuntil the melt forming the part being molded in the cavity 12 is curedsufficiently to retain its shape when removed from the mold.

During the mold open time, the mold is unlocked and opened and themolded part is removed. The mold open time need only be long enough topermit such removal, after which the mold is reclosed and locked-up,ready for a new cycle.

The present invention concerns only the primary pressure on time, whichis divided into two periods referred to as the fill time and thecompaction time, the division being accomplished by process timer 45,energized through primary injection timer 22. In the examples shown inFIGS. 2 and 3, the fill time might be typically taken as 3.5 sec., withthe compaction time occupying the remainder of the 6 sec. primarypressure on time, or 2.5 sec.. The primary injection pressure isreplaced by independent fill and compaction pressures delivered tocylinder 18 during the fill and compaction times respectively.

When the mold 12 is closed and locked-up, a lock-up signal is deliveredfrom the machine 10 to the primary injection timer 22, the positionsample timer 37 and the pressure sample timer 42 to start timing. Theprimary injection timer immediately energizes process timer 45, whichtransfers the fill pressure signal to the programmable power supply 46.The power supply delivers a valve control signal, responsive to the fillpressure signal, to the electro-hydraulic relief valve 30, whichpartially closes, restricting return of hydraulic fluid 27 to sump 28and so building up hydraulic pressure in cylinder 18. The build up inpressure in the cylinder is slowed by the flow restricting valve 29.Pressure in cylinder 18 moves piston 19 and the connected ram 16forward, extruding melt from extrusion barrel 15 through the nozzle 20and injecting it into cavity 12.

As the ram moves forward the coupled wiper 32 advances acrosspotentiometer 31 from the grounded end toward the end connected to avoltage supply V, producing a position signal proportional to ramposition, which signal is delivered to position comparator 35. Aposition setpoint signal is supplied from an adjustable positionsetpoint means 36 to the position comparator, the setpoint signal beingequal to the position signal at a predetermined position of the ram. Ata predetermined time after the lock-up signal has been received, theposition sample timer 37 transmits a position sample signal to theposition comparator 35. If, when the position sample signal is received,there is a difference between the position and position setpointsignals, a ram position deviation signal is delivered to the fillpressure control 38. A fill pressure reference means 39 provides a fillpressure reference signal to the fill pressure control, which sums theram position deviation and fill pressure reference signals to providethe fill pressure control signal. The ram position deviation signal isof such a value that, when added to the fill pressure reference signal,it will produce a fill pressure control signal that will modify thehydraulic pressure delivered to cylinder 18 sufficiently to correct thedeviation in ram position on the next cycle. The fill pressure controlsignal then becomes the new fill pressure reference signal for the nextcycle. The initial fill pressure reference signal is set manually, as bya potentiometer, in accordance with prior experience.

The pressure in the cavity 12 is sensed as by a strain gauge or otherpressure transducer 33, which provides a proportional mold pressuresignal to pressure comparator 40.

A mold pressure setpoint signal is supplied from an adjustable moldpressure setpoint means 41 to the pressure comparator, the setpointsignal being equal to the mold pressure signal produced by apredetermined mold pressure desired to be held at a preestablished timeafter mold lock-up. At said preestablished time after the lock-up signalhas been received, the pressure sample timer 42 transmits a pressuresample signal to the pressure comparator 40. If, when the pressuresample signal is received, there is a difference between the moldpressure and mold pressure setpoint signals, a pressure deviation signalis delivered to the compaction pressure control 43. A compactionpressure reference means 44 provides a compaction pressure referencesignal to the compaction pressure control, which sums the pressuredeviation and compaction reference signals to provide the compactionpressure control signal. The initial compaction pressure referencesignal is set manually, as by a potentiometer, in accordance with priorexperience. Thereafter the compaction pressure control signal becomesthe new compaction pressure reference signal for the next cycle.

In a preferred embodiment, the position and mold pressure setpoints 36,41 cover a range of values defined by upper and lower limits. Thecomparators 35, 40 then provide deviation signals only when thetransducers 31, 33 provide signals above the maximum or below theminimum limits. If the upper and lower limits coincide, there is but onereference value from which deviations are measured. When upper and lowerlimits are employed, the deviation signal produced is usually apredetermined step so that the pressure adjustments are made upward anddownward in correspondingly predetermined increments.

FIG. 4 shows the relationship between fill and compaction pressures, andposition and pressure limits as they apply to the low melt viscositymaterial in FIGS. 2 and 3. Fill pressure on time is set to end when moldpressure begins to build up more rapidly which marks the beginning ofcompaction of the melt in the cavity 12. The substantially linearforward motion of the ram may continue after the fill pressure isreplaced by compaction pressure due to the relatively small amount ofcompaction initially occurring. The position limits are set at somepoint along, but preferrably near the end of, the substantially linearportion of the position curve, regardless of whether they fall withinthe fill or compaction pressure times. The pressure limits are set atsome point after compaction begins, but before the mold pressure fallsappreciably after reaching its peak. Preferrably the pressure limits areset near the peak mold pressure, regardless of whether they fall withinthe compaction or secondary pressure times. In FIG. 4 the positionlimits are shown in the compaction pressure time at the end of thesubstantially linear portion of the position curve. The pressure limitsare shown at the end of the compaction time before the peak moldpressure is reached. The curves in FIG. 4 are the same as the low meltviscosity curves in FIGS. 2 and 3. When the limits are at, or beyond,the ends of the periods during which the pressures to be controlled areapplied, the correction, if any, occurs on the next molding cycle. Thereis some lag in the application of the controlled hydraulic pressures tothe piston 19 resulting from mechanical delay encountered in theelectro-hydraulic relief valve 30 and hydraulic delays introduced by therestriction introduced by flow valve 29. It is primarily the result ofsuch delays that the peak mold pressure falls in the secondary pressureon time.

The embodiment shown and described is only a preferred example of theinvention and does not define the scope of the invention which islimited solely by the claims.

I claim:
 1. A method for control of a process for injecting into andcuring in a mold cavity a plasticized material comprising the steps ofsensing at a predetermined time after initiation of injection theposition of a ram injecting said plasticized material into the cavity,determining the deviation of said sensed position from a predeterminedsetpoint position, adjusting a fill pressure in a manner to compensatefor said position deviation, and compacting the plasticized material insaid cavity under an independent compaction pressure, wherein saidcompacting step comprises sensing the mold pressure in said cavity at asecond predetermined time after initiation of injection and aftercompletion of filling of the cavity, determining the deviation of saidsensed mold pressure from a predetermined setpoint pressure, andadjusting said compaction pressure in a manner to compensate for thepressure deviation.
 2. A method for control of a process for injectinginto and curing in a mold cavity a plasticized material comprising thesteps of filling the mold cavity with said plasticized material under apredetermined fill pressure, sensing the mold pressure in said cavity ata predetermined time after initiation of said filling and aftercompletion of filling of the cavity, determining the deviation of saidsensed mold pressure from a predetermined setpoint pressure, andadjusting a compaction pressure in an independent manner to compensatefor said pressure deviation.
 3. A method according to claim 2 whereinsaid predetermined time is set to occur prior to substantial reductionof pressure in the mold.
 4. A method according to claim 2 wherein saidpredetermined time is set to occur substantially at the maximum pressurein the mold.
 5. A method according to claim 2 wherein said setpointpressure is defined by an upper limit and a lower limit.
 6. A methodaccording to claim 2 wherein said compaction pressure is adjustedupwards and downwards in predetermined increments.
 7. A method accordingto claim 2 further comprising employing said adjusted compactionpressure as the compaction pressure to be adjusted in a followingcontrol cycle.